Soil and groundwater contamination in remote areas is a common occurrence. For instance, contamination can result from leaks in oil and gas pipelines, fuel leaks along railway lines and at maintenance yards and at remote communities and farms. Contaminants may collect in soil either above or below the local water table. In low lying ground, the water table may be near or at the surface of the soil, as in a bog or marsh, and may collect in pondwater, or below the surface of the water in a mud, sand or other type of bottom.
Contaminants of soil and groundwater may include volatile organic compounds, such as for example, trichloroethane, trichloroethylene, benzene, toluene, ethylbenzene, and xylene, hydrocarbon products, such as for example, diesel fuel and kerosene, other petroleum products, such as for example, gasoline, heating oil and lubricating oils and metals, such as for example, iron, manganese and copper.
A number of remediation technologies can be used to combat soil and groundwater contamination. Soil vapour extraction is a process that physically separates contaminants from the soil in vapour form. Soil vapour extraction systems target contaminants that have a tendency to volatilize or evaporate easily, such as volatile organic compounds. This process is known as volatilization. By applying a vacuum through the subsurface, at a level above the local water table, contaminants are drawn to the surface as vapour or gas and are subsequently treated using carbon adsorption, biofiltration, incineration, condensation or catalytic oxidation. Such a treatment technique requires the installation of vapour extraction wells and air injection wells or air vents in the contaminated area.
Air injection wells use air compressors to force air into the ground. Air vents serve the same function but merely provide a passage for air to be drawn into the ground rather than requiring the active pumping of air into the site. As will be discussed below, used alone, soil vapour extraction tends not to be suitable for removing contaminants in the saturated zone of the subsurface, the soil that lies below the water table. In such situations, the process of air sparging may be used in combination with the soil vapour extraction system.
Dual phase extraction is a process that is similar to soil vapour extraction but the extraction wells are sunk further into the ground to a level below the water table and therefore into the saturated zone. A strong vacuum is applied through the extraction wells to remove vapours. A pump is used to remove liquid that collects in the bottom of the extraction well, or wells. Both the extracted gases and liquids can be treated and discharged.
Air sparging and venting are remediation technologies that are used to treat contaminants, such as dissolved volatile organic compounds, in groundwater and soil. Both venting and air sparging involve the injection of air under pressure into soil and groundwater respectively. Specifically, air sparging refers to the injection of air or other gas under pressure at a level below the height of the local water table into the saturated zone. The injected air displaces water and creates air-filled porosity or channels in the saturated soil. Venting refers to the injection of air or gas into the subsoil at a level above the height of the local water table, that is, into the unsaturated zone. To work, air sparging and venting rely on two basic mechanisms, biodegradation and volatilization. Biodegradation is typically stimulated by the introduction of oxygen in air into the site to enable naturally occurring microorganisms to biologically degrade organic compounds into harmless substances. Volatilization occurs when the contaminant, or a portion of the contaminant, is encouraged to evaporate, typically by the provision of a gas in which the contaminant vapour can be carried away.
Air sparging can also be used to change the electrochemical potential of the groundwater from a reducing potential to an oxidizing potential. Groundwater is frequently found to be under reducing conditions which can cause increased concentrations of some metals including, but not limited to, iron, manganese, arsenic, selenium and copper. By introducing oxygen or air into the groundwater, the electrochemical potential will tend to be changed from reducing to oxidizing and the metal ions will be oxidized to less soluble forms. For example, soluble Fe.sup.2+ will tend to oxidize to Fe.sup.3+ which will tend to precipitate out as iron hydroxide.
Engineered biocells or biopiles can also be used for soil remediation. This technology involves excavating contaminated soil and forming it into piles. Slotted or perforated piping is placed to extend through the piles. Biopiles use oxygen, usually from air, to stimulate the growth of aerobic bacteria which degrade certain contaminants, such as petroleum products adsorbed to soil. Biopiles are aerated by forcing air to move by injection or extraction through the perforated piping placed throughout the piles. As with the other remediation systems discussed, biopiles remove soil contaminants through the processes of evaporation and biodegradation.
The effectiveness of many of the remediation techniques described above can be increased through the use of pulse systems. Injection of pulses of gas into the ground and groundwater provides for better mixing and increases the density of uniformly distributed air channels. Typically, pulse systems are set up using an electrical timer or a programmable logic controller, or both, either to turn an air blower on and off as required, or to operate control valves for directing pressurized air to several different wells in a programmed sequence. The pulsing system will turn the air flow to a well `On` for the length of time required to have the soil and groundwater reach an equilibrium condition, where the oxygen levels have reached a maximum concentration or the concentration of volatile organic compounds reaches a steady state. Typically, the `On` cycle will last from 30 minutes to eight hours depending on the contaminant, subsurface conditions, air flow rate and air pressures. The pulsing system will turn the air flow `Off` when equilibrium conditions have been reached because additional flow is of limited value and the system will redirect the flow to another well or shut-off the blower to save energy. The `Off` cycle will last until the oxygen levels have dropped too low or volatile contaminant concentrations have increased. Typically the `Off` cycle will last from 30 minutes to several days.
Gasoline and diesel engines, and electrical motors can be used to drive air pumps, blowers, compressors or similar equipment to inject air directly into contaminated soils and groundwater, whether in a sparging process or in a venting process. As well, compressed gas such as compressed air or liquid oxygen can be used to provide pressurized air or oxygen to a subsurface well. The compressed air or liquid oxygen is then released through vents in the well and travels into the soil and groundwater. Similarly, depending on the remediation process chosen, vacuums can be used to vaporize and extract contaminants from the site. Both fossil fuel driven generators and electrical line power brought in from a utility offer immediate, on demand response to system requests for power either to provide compressed air or to operate vacuum extraction equipment.
These current soil and groundwater contamination systems have a number of disadvantages. Among the disadvantages of electric motors is the need to bring in electricity from a significant distance. This may not be practical at remote locations where power is not available from a local electric utility, or is only available at unreasonable expense. In those instances it would be advantageous not to have to draw power from an electrical grid.
One alternative to this is to use a fossil fuel driven system, such as a diesel or gasoline engine, either to drive a generator to provide power for an electric motor, or directly to drive a compressor or similar machine. However, such equipment requires refuelling relatively often, and may require relatively frequent visits by an operator or maintenance technician. The equipment itself may contribute to the kind of soil remediation problem that sparging and bioventing are intended to alleviate. That is, in some remote locations, soil contamination problems have arisen due to the abandonment of fuel and lubricant materials originally used to operate engines for generating electrical power. It would be advantageous to have a soil remediation system that requires relatively little outside intervention, whether in terms of the need for refuelling, the need for maintenance, or the need to clean up after the remediation process itself is finished. It would be advantageous to dispense with the need for a fossil fuel engine.
The terms "passive energy conversion system" and "passive gas recharging system" are used to refer to a system that does not provide power on demand. That is, passive systems rely on casual energy capture, and do not provide predictable, on demand power in the manner of a gasoline powered generator or an electrical motor drawing power from the power grid of an electrical utility. A passive energy conversion system is amenable for use with remediation systems, such as air sparging, because such remediation systems do not have inflexible time frame demands. In other words, the effectiveness of a remediation system is not dependent on a fixed duty cycle. On the contrary, soil remediation projects are frequently highly time insensitive. Remediation may occur over a period of months or years. Taken over the longer time period, the energy requirement of remote soil remediation systems may fall within the range of that which is available on a passive basis either from wind power, solar power, or some other passive energy source. While these passive energy sources may be relatively unpredictable on an instantaneous demand basis, their overall, average power availability is suitable to a process, such as soil remediation, that may have a relatively forgiving time scale. Passive energy conversion systems such as wind, solar, wave, geothermal and hydraulic energy conversion systems are amenable for use in remote areas where electrical power may not be readily available from an electrical utility.
On an instantaneous demand basis, a passive energy conversion system such as a windmill, for example, taken alone, may only be able to produce low flow rates and short pulses. A windmill, or solar collector, may be unable to produce adequate pressure and pulse rate to permit effective air sparging and venting of the contaminated site on an on demand basis. However, when coupled with a pulse or charge storage device, it may be possible to achieve satisfactory soil and groundwater remediation results.
With a view to overcoming these problems it would be advantageous to provide an apparatus and method to effect soil and groundwater remediation by accumulating a gas to a sufficient pressure using a passive energy conversion system such as a windmill, for example, and a pressure tank, and discharging a pulse of pressurized gas into contaminated soil and groundwater. Alternatively, it would be advantageous to operate a vacuum extraction system using a passive system.